Most, and probably all murine strains, contain within their genomes clusters of genes that can be induced to form RNA tumor viruses. These so-called endogenous viruses are involved in oncogenesis and in some cases of auto-immune disease. They are also hypothesized to play a role in normal development. We are attempting to analyze the organization of these endogenous virogene sequences in the AKR/J, Balb/c and NZB strains of mouse by the application of two-dimensional chromatography and Southern blot analysis. In addition we are analyzing cloned chromosomal DNA fragments containing endogenous virogene sequences derived from the AKR/J mouse.